The present invention relates to self-processing photographic film units, sometimes called instant film units, and to mechanisms for controlling the depth and uniformity of a layer of processing fluid distributed in such units between two sheets.
Self-processing film units typically include a pouch of processing fluid coupled to first and second superposed sheets. One of the sheets supports photosensitive material for recording a latent image that is processable by the fluid to establish a visible print. The other sheet serves as a cover to confine the fluid and to facilitate its distribution from the pouch over the photosensitive material.
The fluid is distributed by applying compressive pressure to the film unit progressively from one end to the other. This ruptures the pouch, expels the fluid between the sheets and drives it in a wave toward the opposite end of the film unit where any excess is collected in a trap. Although the distributed fluid may be quickly absorbed, it exists for some finite time as a layer which has a depth suitable, when absorbed, to carry out the processing function. It is common to measure this fluid depth, and to consider its uniformity, when analyzing related photographic effects.
Compression-resistant elements are usually employed to maintain a spacing between the sheets and thereby control the depth of the fluid layer when the compressive pressure is applied. According to one well known approach, elongate strips called side rails are provided along the lateral edges of the film unit from the pouch to the trap. Alternatively, a mask, which frames the imaging area, can serve the additional function of controlling the spacing between the sheets. Although many variables affect the depth of the distributed fluid, spacing elements such as those mentioned above, are the most influential.
The uniformity of the depth of the fluid layer is also important. The fluid tends to flow in a tongue-shaped wave ahead of the pressure mechanism. This moves more fluid down the middle of the film unit than at its edges, sometimes resulting in uneven processing. For these and other reasons, which are known to those skilled in the art, it is common to modify the distribution to direct more fluid toward the lateral edges of the picture area. In some film units the fluid pouch is sectioned or sealed in a pattern which provides more fluid at the lateral edges. In other cases the processing mechanism includes surfaces which abut the film unit near the leading end of the tongue-shaped wave to divert some of the fluid toward the lateral edges.
The above background is only representative of the numerous approaches that have been proposed for controlling the depth and the uniformity of the depth of a processing fluid layer in self-processing film units. It will become apparent from the following description, however, that none of the prior approaches offer the important advantages of the present invention.